Method of making a semiconductor having selectively enhanced field oxide areas

ABSTRACT

A field oxide is provided which purposefully takes advantage of fluorine mobility from an implanted impurity species. The field oxide can be enhanced or thickened according to the size (area and thickness) of the oxide. Fluorine from the impurity species provides for dislodgement of oxygen at silicon-oxygen bond sites, leading to oxygen recombination at the field oxide/substrate interface. Thickening of the oxide through recombination occurs after it is initially grown and implanted. Accordingly, initial thermal oxidation can be shortened to enhance throughput. The fluorine-enhanced thickening effect can therefore compensate for the shorter thermal oxidation time. Moreover, the thickened oxide regions are anistropically oxidized underneath existing thermally grown oxides and directly underneath openings between nitrides. The thickened oxides therefore do not cause additional shrinkage of the active areas which reside between field oxides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor processing and more particularlyto patterned field oxides which can be selectively thickened accordingto size and area of the oxides.

2. Background of the Relevant Art

A bare silicon surface will generally oxidize in air at roomtemperature, but because the growth is self-limiting, only a relativelysmall thickness can be achieved. The native oxide which is formed,provides a protective coating which prevents corrosion and preventsfurther oxidation in most room-temperature ambients. When thetemperature is raised several hundred degrees C, the oxidation rate ismuch greater, and the oxide becomes denser and more durable.Accordingly, thermal oxidation of silicon has become a mainstay in theformation of oxides within active regions (gate oxides) and oxideswithin inactive regions (field oxides).

High temperature thermal oxides are formed by the direct oxidation ofthe silicon wafer surface at elevated temperatures in either a dryoxygen or a steam ambient. The resulting oxide is relatively imperviousto diffusion species for at least as long as the time required for thediffusion to occur. Furthermore, the oxide is relatively free ofpinholes. Most of the Group IIIA and VA dopants react with the thermallygrown oxide to form silicates which gradually move through the oxide.Generally speaking, silicates do not form completely through and do nottouch the underlying silicon and, therefore, do not become a diffusionsource for the active or inactive regions. Thus, the thickness of theoxide must be carefully tailored to the required diffusion time andtemperature.

While other materials such as chemical vapor deposition (CVD) oxide ornitride can be used as a mask against diffusion species, no commerciallyproven substitute for a thermal oxide currently exists to providesilicon pn junction passivation or MOS gate oxides. The prime importanceof thermal oxide stems from its ability to properly terminate thesilicon bond at the silicon-oxide interface. The deleterious electricaleffects of a bare silicon surface, or of one covered only withroom-temperature native oxides are thereby minimized with thermallygrown oxides.

In order to form active and inactive regions, a process known as localoxidation of silicon (LOCOS) has been used to provide dielectricisolation. LOCOS utilizes a silicon nitride material (i.e., Si₃ N₄) tomask active regions from the growth of field oxide. Being somewhatimpervious to oxygen diffusion, the silicon nitride layer is placed uponbare silicon, or upon a pad oxide overlying bare silicon, at selectregions upon the wafer. A subsequent oxidation step will thermally growoxide predominantly where the nitride is not present. The oxidation rateupon the upper surface of the nitride is much slower than the rate uponthe bare silicon between the nitrides. As such, the nitrides and smallthin layer of oxide at the upper surface of the nitride is easilyremoved leaving thick field oxides in the non-masked areas.

Because of the high stress at the nitride-silicon interface,dislocations will often be generated in the silicon at the edges of theoxide (i.e., at the edges of the non-nitride windows). Interfacedislocations and/or stress often exasperates the problem of lateraldiffusion of oxygen within the silicon. Lateral diffusion of oxygen orlateral consumption of silicon by growing oxide at the nitride interfacegives rise to "birdbeak" structures. Generally speaking, the densityvalue reported for thermal oxide is close to that of fused silica and isalmost the same as that of silicon. These densities, coupled withrespective molecular weights of 60 and 28 (molecular weight of oxide andsilicon, respectively), dictate that for every volume of siliconoxidized, 2.2 volumes of oxide will be generated. Silicon will thereforebe consumed and the top of the oxide will rise above and extend outwardfrom the original silicon surface.

As the density of integrated circuits increase and circuit dimensionsdecrease, the problems of lateral growth of field oxides are compounded.Furthermore, high density processes bring about another problemgenerally referred to as "field oxide thinning." Field oxide thinningand birdbeak formation is best described by referencing FIGS. 1 and 2.In FIG. 1, a cross-sectional view of a semiconductor substrate 10 isshown. Substrate 10 includes an upper surface adapted to receive amasking layer, such as silicon nitride, necessary for LOCOS processing.Silicon nitride is generally blanket deposited across the entiresubstrate and then etched to present a patterned silicon nitride 12.Windows, which are etched within the nitride layer to form pattern 12,can be either small or large area windows, A1 or A2, respectively.

Thermal oxides can be grown in either atmospheric or high pressurechambers. Atmospheric equipment cannot achieve the higher oxidation rateof high pressure equipment, however, atmospheric units are lessexpensive to operate and maintain. In either case, oxidation equipmentforwards an oxidizing agent such as dry oxygen or steam across the uppersurface of substrate 10. It is well recognized that growth resultingfrom the oxygen or steam will vary depending upon the size of thesilicon surface presented to the ambient. The growth rate in confinedspaces or areas, such as area A1, is less than growth in larger areas,such as area A2. Furthermore, the oxide grown in a confined area usuallyhas an increased stress within the resulting product. Accordingly, FIG.2 illustrates varying growth rates in directions lateral andperpendicular to substrate 10 upper surface. For a given oxidation time,thickness t1 is shown less than thickness t2. Birdbeak areas 18 occur tofurther lessen active areas between field oxide 20 and 22.

Growth of field oxides 20 and 22 depend upon many factors, including:oxidation time, oxidizing ambient, ambient pressure and/or temperature,impurity in the silicon, the amount of stress in the oxide and silicon,and area of silicon exposed to the ambient. Oxide thinning is the resultof oxides growing at a slower rate in small areas than in larger areas.The result of small oxide growth, or oxide thinning, is that of asmaller dielectric thickness t1 in some field areas than inothers--i.e., thickness t2 of large oxide 22. A thinner dielectric, whencoupled with an overlying conductor, presents parasitic capacitance. Ifthe dielectric is small enough, field inversion beneath of thinnerdielectric will occur. High density integrated circuits further compoundthe field thinning effect and can lead to "turn-on" channels in areaswhere a channel should not exist. Such problems can lead toinoperability of the resulting circuit.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by the improvedfield oxide structure and method of manufacture accorded to the presentinvention. That is, field oxide regions which are small in area can beenhanced to offset the field oxide thinning effect. Oxides are thickenedafter they are thermally grown by doping the oxides with an impurityspecies which includes fluorine, and allowing the implanted fluorine todislodge oxygen from the oxide bulk. The dislodged oxygen recombine withsilicon at the juncture between the oxide and the substrate to formsilicon dioxide bonds therein.

The thickened field oxide occurs predominantly in small oxide areas. Alarger amount of fluorine will migrate into and through thin oxides thancounterpart thick oxides. As a result, a larger portion of fluorineatoms exist at or near the interface of a small oxide area necessary fordislodgment and subsequent recombination at the interface. Thin oxidestherefore present a naturally receptive area for the enhanced orthickened effect caused by recombination. An important advantage thereofbeing a substantial equilibrium or uniformity in thickness of the final.oxides formed across the wafer as a result of recombination, regardlessof the initial, thermal oxidation-formed size. The thin oxides willoxidize further into the substrate as a result of fluorine mobility inorder to bring their thickness up to and substantially equal to thickeroxides of larger area. Blanket implanted impurity species comprisingfluorine will therefore provide a natural evenness to oxides of varyingsize which have been initially grown to an uneven thickness. As definedherein, "impurity species" includes any ionic chemical compound orelement having fluorine as an element within the compound or as the soleelement. Exemplary impurity species include: BF₂ +, BF+, or F+. A BF₃gaseous source material may be used to produce the source to an ionimplanter equipped with an analyzer magnet for passing only the selectionic output (e.g., BF₂ +, BF+, or F+).

Broadly speaking, the present invention contemplates a method forthickening a small oxide greater than a large oxide. The methodcomprises the steps of providing a substrate and a nitride layerpatterned across the substrate. A small oxide of relatively small areaand thickness as well as a large oxide of relatively large area andthickness is grown upon the substrate and between the patterns of thenitride layer. Next, an impurity species, which includes fluorine, isimplanted within the small oxide and large oxide. A greater amount offluorine from the species will diffuse through the small oxide thanthrough the large oxide, resulting in greater thickening of the smalloxide than the large oxide.

Without being bound to theory, the mechanism by which fluorine causes athickening of the small oxide begins by understanding the high mobilityof fluorine, and the ability of fluorine to quickly diffuse to aplurality of silicon-oxygen bond sites located within a bulk portion ofthe oxide, near the silicon/oxide interface. The fluorine ions candislodge oxygen atoms at the plurality of bond sites, wherein thedislodged oxygen atoms recombine with silicon atoms at the interface orjuncture between the small oxide and substrate. Converse to small oxidedislodgment and recombination processes, large oxide, due to its initialthickness (i.e., thermally grown thickness), substantially prevents thedislodgment and recombination steps of a substantial majority of thefluorine atoms. In other words, dislodgment of oxygen and therecombination of the oxygen at the interface occurs less frequently andin smaller amounts in large oxides than in small oxides.

The present invention further contemplates a method for increasing thethickness of a small oxide to substantially match the thickness of alarge oxide. The method comprises the steps of providing a siliconsubstrate having a substantially planar upper surface, and placing alayer of nitride upon the upper surface. The nitride is patterned with aplurality of windows extending through the nitride to the upper surface.At least one of the windows comprises a small area extending parallel tothe upper surface and another of the windows comprises a large area alsoextending parallel to the upper surface. The small area is relativelysmaller than the large area. A small oxide of small thickness is grownwithin the small window simultaneous with a large oxide grown within alarge window. Both the small and large oxides grow perpendicular andparallel to the upper surface. After the small and large oxides aregrown, an impurity species is implanted within the small oxide and largeoxide. Fluorine from the species will diffuse to a deeper depth orthrough the lesser thickness of the small oxide to a greater extent thaninto or through the greater thickness of the large oxide. The substrateis then oxidized with the aid of the fluorine in a directionsubstantially perpendicular to the upper surface directly beneath thesmall window and the large window. The substrate underlying the smalloxide is oxidized at a faster rate than the substrate underlying thelarge oxide. The oxidizing step is allowed to continue for a sufficienttime to permit the thickness of the small oxide to substantially matchthe thickness of the large oxide.

The enhanced thickness of the small oxide lies directly underneath thesmall window, and therefore, does not extend laterally into adjacentactive areas--a deleterious effect often attributed to birdbeakformation. By overcoming normal lateral birdbeak formation, the presentoxidizing step for providing additional oxide, or enhancing oxidethickness, is well suited for high density circuit applications.Moreover, the resulting enhanced or thickened small oxide is lesssusceptible to field inversion or turn-on caused by high voltages orcurrents within overlying conductors. Still further, the impurityspecies advantageously includes BF₂, which can provide fluorine(recombination enhancement feature) as well as boron (channel stopdopant feature). Boron is a dopant which is placed into the substrateunderlying the oxides. More boron dopant will diffuse through the smalloxide than through the large oxide. Accordingly, boron dopant as well asthickened oxide prevents inversion of the channel region. Introductionof boron increases minority carriers in p- substrate tubs and does notsignificantly decrease heavily doped n- tubs. Thus, boron effectivelyincreases the threshold voltages in any CMOS-processed channel. BF₂ ionsthereby provide a dual function which is an important aspect of thepresent invention.

The present invention still further contemplates an enhanced field oxidearea. The area comprises a silicon substrate having a planar uppersurface and an oxide having silicon-oxygen atomic bonds grown upon theupper surface. Fluorine atoms can be anistropically placed within theoxide, wherein a substantial amount of fluorine atoms migrate anddislodge oxygen from the silicon-oxygen bonds. Additional oxygen at thejuncture between the oxide and the upper surface resulting from thedislodged oxygen can recombine with silicon atoms within the substrate.Due to the anistropic nature of fluorine implant, oxygen recombinationsites exist only within an area directly beneath the windows and betweenthe patterns of nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a cross-sectional view of a semiconductor substrate and apatterned nitride layer placed upon the upper surface of the substrateaccording to one step of a prior art manufacturing process;

FIG. 2 is a cross-sectional view of small and large area field oxidesgrown upon the substrate and between the patterned nitride according toanother step of a prior art manufacturing process;

FIG. 3a is a cross-sectional view of small and large area field oxidesimplanted with fluorine ions during a processing step according to anexemplary embodiment of the present invention;

FIG. 3b is a cross-sectional view of small and large area field oxidesimplanted with BF₂ ions during a processing step according to anotherexemplary embodiment of the present invention;

FIG. 4 is a cross-sectional view of small and large area field oxideswhich are enhanced or thickened according to another processing step ofthe present invention; and

FIG. 5 is a cross-sectional view of enhanced small and large fieldoxides with overlying metallization conductors formed according to yetanother processing step of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments are shown by way of example inthe drawings and will herein be described in detail. It should beunderstood, however, that the drawings and description thereto are notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents and alternatives falling within the spirit and scope of thepresent. invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 3a, a cross-sectional view of small oxide 20 andlarge oxide 22 is shown. Oxides 20 and 22 are thermally grown uponsubstrate 10 and between patterned nitride 12. The upper topography ofthe structure shown in FIG. 3a can, according to one embodiment, beimplanted with fluorine ions using conventional implantation techniques.Although not shown, but well understood, the fluorine ions are implantedafter a channel stop implantation step. Channel stop implant generallyinvolves masking, for example, the P- well with photoresist and dopingthe field oxides 20 and 22 with boron. The boron dopant is introducedafter nitride deposition to prevent boron from entering the activeregions. Accordingly, FIG. 3a illustrates boron atoms placed in theinactive regions as indicated by reference numeral 24. The boron atomshelp minimize occurance of field oxide inversion.

Preferably, fluorine and/or boron ions are anistropically implanted witheither a low or medium current ion implanter. The implanter ionizes anappropriate molecular species and filters the ionized species accordingto atomic mass number. A resulting ion beam is then focused upon theupper topography surface of the substrate. The fluorine ions areaccelerated into the substrate at a dose within the range of 1×10¹³atoms/cm² to 5×10¹⁴ atoms/cm² at an implant energy greater than 100 keV,for a 3000 Å field oxide. The depth of implant is within the range of2010 Å, with a lateral straggle (or std. deviation) of approximately 640Å. Likewise, boron ions can be accelerated upon the substrate at a dosewithin the range of 1×10¹³ atoms/cm² to 6×10¹³ atoms/cm² at an implantenergy of approximately 26 keV. The depth of implant is approximatelywithin the range of 943 Å.

Implantation of fluorine ions has been carefully studied in recentyears. It has been determined that fluorine ions, when implanted into apolysilicon material, can cause thickening of the underlying gate oxideSee, e.g., Wright, et al., "The Effect of Fluorine in Silicon DioxideGate Dielectrics," IEEE Trans. on Electron Devices, vol. 36, no. 5, May,1989; and, Sung, et al., "A Comprehensive Study on p+ Polysilicon-GateMOSFET's Instability With Fluorine Incorporation," IEEE Trans. onElectron Devices, vol. 37, no. 11, Nov., 1990 (incorporated herein byreference). Fluorine atoms readily migrate to weakened bonds within thepolysilicon and forces oxygen to recombine with silicon at the gateoxide/silicon interface. A thickened gate oxide will thereby increaseturn-on voltages as well as gate oxide capacitance. In order toalleviate the problem, recent work has focused upon co-doping POCl₃within the polysilicon to block fluorine mobility and thereby lessen orminimize the recombination effect. Work has thereby focused upon tryingto minimize or prevent fluorine mobility instead of encouragingmobility. See, Hsieh, et al., "Characteristics of MOS Capacitors of BF₂or B Implanted Polysilicon Gate With and Without POCl₃ Co-doped," IEEEElectron Device Letters, vol. 14, no. 5, May, 1993.

As shown in FIG. 3a, fluorine ions are placed within small and largefield oxides 20 and 22, respectively, to purposefully increase orenhance the recombination effect. In addition to fluorine, boron canalso be used to provide a channel stop feature (i.e., to prevent fieldoxide inversion). Turning now to FIG. 3b, a cross-sectional view ofsmall and large oxides 20 and 22, respectively, are shown implanted withrelatively high energy BF₂ ions according to an alternate embodiment.BF₂ implant is used to encourage fluorine mobility within the oxides aswell as provide boron dopant underneath the oxides. Thus, BF₂ can beused to provide a dual purpose. Instead of having to undergo a priorlithography (masking) step with boron implant, boron from BF₂ can beimplanted simultaneous with fluorine. However, it is understood that thestarting concentration density of the N- well may require pre-adjustmentin order to compensate for additional boron (n- type impurities) at theN- well/field oxide interface. Fluorine, from the BF₂, readily migratesdeep within the oxides, or through the oxides, and is purposefully usedto dislodge oxygen from silicon-oxygen bonds. Furthermore, the dislodgedoxygen, generally found near the interface region or near the underlyingsilicon, has a lesser distance to travel to the interface region whereit recombines with silicon therein. Thus, highly mobile fluorine is morepredominant in small oxide 20 having a thinner dimension t1 than largeroxide 22 having a thicker dimension t2.

Blanket F+ or BF₂ + implant is anistropically placed within the oxidesuch that the silicon-oxygen bonds, which are broken by the mobilefluorine, are directly beneath the windows which separate patternednitride 12. As shown in FIG. 4, anisotropic implantation and resultingoxygen recombination occurs in regions 25 and 26, adjacent small andlarge oxides 20 and 22, respectively. Region 25 is only formed directlybelow window 28, while region 26 is only formed directly below window30. Very little if any lateral diffusion occurs during the formation ofadditional oxides in region 25 and 26. Advantageously, additional oxidewithin regions 25 and 26 do not add to birdbeak problems and shrinkageof active regions between oxides.

Additional oxide within region 25 has a greater thickness thanadditional oxide within region 26, the reason being that more fluorineatoms permeate substantially through or completely through oxide 20 thanoxide 22. More oxygen will thereby recombine within region 25 thanregion 26. Fluorine mobility, oxygen dislodgment, and oxygenrecombination hereof is therefore well suited to enhance the growth rateof additional oxides such that, over time, the final smaller oxidethickness will substantially match the final larger oxide thickness.

Still further, boron dopants from BF₂ ions are implanted through oxides20 and 22, provided suitable implant energy and concentration ispresent. Boron dopants (either originated separate from and prior tofluorine dopants, as in FIG. 3a, or simultaneous with fluorine dopants,as in FIG. 3b) are placed into the substrate and accumulate into channelregion underneath the field oxides, as shown by reference numeral 32.For simplicity, dopants 34 are shown extending across the entirety offield oxide 22 and into the N- well, even though it is well recognizedthat they would not extend the entire distance if the N- well is masked.Additional boron concentration 32 can help reduce field inversionproblems normally associated with thinner field oxides. Thus, if BF₂ions are used, the BF₂ ions provide a dual purpose: (i) to provide anadditional oxide growth rate inversely proportional to the size of theinitial, thermally grown oxide, and (ii) to dope the substrateunderlying the oxides to a concentration level inversely proportional tothe size or thickness of the oxide.

Referring now to FIG. 5, a cross-sectional view of active and inactiveregions of a portion of a semiconductor surface are shown. Inparticular, an enhanced small field oxide 36 (a combination of initialoxide 20 and additional oxide 25) as well as an enhanced large fieldoxide 38 (a combination of initial oxide 22 and additional oxide 26) isshown. The enhanced small and large oxides 36 and 38, respectively,include an upper surface for receiving patterned metallization 40.Metallization 40 includes a conductive strip which can electricallyconnect various circuits formed across the semiconductor die.Metallization 40 (either polysilicon, aluminum, etc.) can also be placedupon gate electrodes residing over gate oxides 42 within active regions46. Gate oxides 42 are much thinner than enhanced field oxides 36 and38. Preferably, gate oxides 42 are less than a hundred Ångstroms inthickness, while field oxides are oftentimes a few thousand Ångstroms inthickness.

By using fluorine to enhance oxygen recombination at select interfacesites, the additional oxides 25 and 26 can be used to offset the thermaloxidation time. In other words, thermal oxidation and time relatedthereto can be reduced or shortened. A shorter thermal oxidation timecan increase wafer throughput. Additional oxides 25 and 25 are thereforeused to compensate for thinner, thermally grown oxides.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that this invention is believed to be capable of PMOS orCMOS applications using either (i) masked boron implant followed byblanket fluorine implant, or (ii) blanket BF₂ implant. In either case,the resulting device is capable of shallow channel and shallowsource/drain operation. It is also to be understood that the form of theinvention shown and described is to be taken as presently preferredembodiments. Various modifications and changes may be made withoutdeparting from the spirit and scope of invention as set forth in theclaims. Provided the impurity species (fluorine or BF₂ ions) are placedin a substantially anisotropic fashion, an exemplary modifications mightbe one which uses various types of implant or diffusion equipment forintroducing fluorine into the oxide. Fluorine ions implanted within therange (depth) and at the energy level specified hereinabove are merelyexemplary dosages and energies for an ion implanter. Numerous otherranges and levels can be achieved provided fluorine is implanted to asufficient depth within the small and large oxides to allow mobility andsubsequent oxidation effect at the small oxide/substrate interface,while substantially minimizing the same effect at the largeoxide/substrate interface. It is intended that the following claims beinterpreted to embrace all such modifications and changes.

What is claimed is:
 1. A method for thickening a first oxide greaterthan a second oxide, said method comprising the steps of:providing asubstrate; thermally growing a first oxide of a first area and thicknesssimultaneous with thermally growing a second oxide of a second area andthickness upon said substrate, wherein said second area and thickness isgreater than said first area and thickness; and implanting an impurityspecies within said first oxide and said second oxide, wherein saidimpurity species comprises a chemical selected from the group consistingof BF₂, BF and F, and wherein fluorine from said species will diffusethrough said first oxide to a greater extent than through said secondoxide resulting in greater thickening of said first oxide than saidsecond oxide.
 2. The method as recited in claim 1, wherein saidimplanting step comprises:allowing a portion of said fluorine atoms todiffuse to a plurality of silicon-oxygen bond sites located within abulk portion of said first oxide near a juncture between said firstoxide and said substrate; allowing said fluorine atoms to dislodgeoxygen atoms at said plurality of silicon-oxygen bond sites; andallowing said dislodged oxygen atoms to recombine with silicon atoms atsaid juncture.
 3. The method as recited in claim 1, wherein saidimplanting step comprises:preventing a majority of said fluorine atomsfrom diffusing to a plurality of silicon-oxygen bond sites locatedwithin a bulk portion of said second oxide near a juncture between saidsecond oxide and said substrate; preventing said fluorine atoms fromdislodging oxygen atoms at said plurality of silicon-oxygen bond sites;and preventing said dislodged atoms from recombining with silicon atomsat said juncture.
 4. The method as recited in claim 1, wherein saidimplanting step comprises allowing a portion of boron atoms from saidimpurity species to reside at a point below said first oxide and saidsecond oxide and within said substrate underlying respective said firstand second oxides.
 5. The method as recited in claim 5, wherein saidboron atoms comprise a dopant source material that, when implanted intosaid substrate, increases threshold voltages within said substrateunderlying said first and second oxides.
 6. The method as recited inclaim 1, wherein said implanting step comprises the steps of:providing awindow through a patterned nitride layer; dislodging oxygen atoms withsaid fluorine in an area within said first oxide directly beneath saidwindow; and recombining said oxygen atoms at a juncture between saidfirst oxide and said substrate directly beneath said window.
 7. Themethod as recited in claim 1, further comprising implanting conductorsover said small oxide and said large oxide.
 8. A method for increasingthe thickness of a first oxide to substantially match the thickness of asecond oxide, said method comprising the steps of:providing a siliconsubstrate having a substantially planar upper surface; placing a layerof nitride upon said upper surface; patterning a plurality of first andsecond windows extending through said nitride and to said upper surface,wherein at least one of said first windows comprises a first areaextending parallel to said upper surface and at least one of said secondwindows comprises a second area extending parallel to said uppersurface, said first area is smaller than said second area; growing afirst oxide of first thickness within said first window in a directionperpendicular and parallel to said upper surface; simultaneously growinga second oxide of second thickness within said second window in adirection perpendicular and parallel to said upper surface; implantingan impurity species within said first oxide and said second oxide,wherein said impurity species comprises a chemical selected from thegroup consisting of BF₂, BF and F; and wherein fluorine atoms from saidspecies will move through said first oxide to a lesser extent thanthrough said second oxide; oxidizing said substrate with the aid of saidfluorine atoms in a direction perpendicular to said upper surfacedirectly beneath said first window and said second window, wherein saidsubstrate underlying said first oxide is oxidized to a greater degreethan said substrate underlying said second oxide; and allowing saidoxidizing step to continue for a sufficient time to permit the thicknessof said first oxide to match the thickness of said second oxide.
 9. Themethod as recited in claim 8, wherein said implanting step comprisesallowing a portion of boron atoms from said impurity species to resideat a point below said first oxide and said second oxide and within saidsubstrate underlying respective said first and second oxides.
 10. Themethod as recited in claim 9, wherein said boron atoms comprise a dopantsource material that, when implanted into said substrate, increasesthreshold voltages within said substrate underlying said first oxide toa greater extent than said substrate underlying said second oxides. 11.The method as recited in claim 8, wherein said oxidizing stepcomprises:breaking silicon-oxygen bonds within said first oxide andadjacent said substrate with said fluorine in a region directly beneathsaid first window; breaking silicon-oxygen bonds to a lesser extent thansaid immediately preceding step within said second oxide and adjacentsaid substrate with said fluorine in a region directly beneath saidsecond window; recombining oxygen atoms at a juncture between said firstoxide and said substrate directly beneath said first window; andrecombining oxygen atoms to a lesser extent than said immediatelypreceding step at a juncture between said second oxide and saidsubstrate directly beneath said first window.
 12. The method as recitedin claim 8, further comprising implanting conductors over said firstoxide and said second oxide.